This invention relates to polyolefin blends. In one aspect, the invention relates to polyolefin blends suitable for solid state processing while in another aspect, the invention relates to particular blends of various polyethylenes. In still another aspect, the invention relates to polyolefin blends characterized by melting and crystallization curves exhibiting either two distinct melt regions, or wherein one component has a softening point lower than that of a conventional polyolefin, either technique of which provides an unusually wide window for solid state processing.
While the field of metal forming offers a wide variety of solid state processing, e.g. stamping, forging, rolling, extrusion, etc., the solid state processing of semicrystalline polymers is much more limited. To process a thermoplastic in the solid state, typically the processing temperature is maintained at just a few degrees below the melting point of the polymer. If the processing temperature is above the melting point of the polymer, then the processing is simply a melt extrusion. At temperatures considerably below the melting point of the polymer, the polymer is essentially a solid and its deformation requires a tremendous amount of pressure (often in excess of one million psi). Such enormous pressures make large deformations very difficult, and result in a relatively low production rate and a generally energy inefficient process. Moreover, since the processing temperature is the most critical parameter in solid state processing, this technique has been limited to those polymers and polymer blends with relatively wide (e.g. at least about 20 degrees on the Celsius scale) ranges between their softening and melting temperatures. Since this temperature range is relatively narrow for conventional polyolefins and their blends, these materials have not been the subject of extensive, commercial-scale solid state processing.
Despite their relatively narrow processing temperature window, the sheer commercial scale of the polyolefin market has generated considerable interest in applying this technique to polyolefins. The potential benefits to be gained include energy efficiency, speed, and scrap reduction. Ciferri and Ward in "Ultra High Modulus Polymers", Applied Science Publications, London (1979), teach that the extrusion of polymers below the melting point is a promising method of producing highly oriented materials. Krjutchkov, et al. in Polymer Composite, Vol. 7, No. 6, pp. 4-13-420 (1986), disclosed the detailed investigations of the dynamics of solid state extrusion, and they have also postulated a model to correlate the flow of instability in extrudate defects to the extrusion temperature and pressure.
Chung in U.S. Pat. No. 5,028,663 teaches that certain blends of high density polyethylene (HDPE) and low density polyethylene (LDPE) prepared by solution mixing can achieve a broader operating window for certain solid state processing applications than would otherwise be achieved by mechanical mixing. The melting peak of each component in the blend is separated due to solution precipitation such that large deformation processing can be performed at a temperature between the melting peaks of the two components.
Pawloski, et al. in U.S. Pat. Nos. 4,352,766, 4,161,502 and 3,739,052 teach a unique process called Solid Phase Forming (SPF) to form polymers and composites at a pre-melt state. This unique fabrication process induces a biaxial orientation throughout the finished parts.
Enikolopow, et al. in U.S. Pat. No. 4,607,797 teach a process called Solid State Sheer Extrusion or Pulverization to perform polymer extrusion at a temperature below the peak melting temperature of the polymer. However, Shutov has observed in a 1992 I.I.T. Research Report that certain polymers, such as HDPE, polypropylene (PP) and polycarbonate (PC) did not perform well in this process.
The disclosures of each of the patents referenced above are incorporated herein by reference.